1. Field
Embodiments relate to a method for producing a cubic boron nitride (cBN) thin film and a cBN thin film structure. More particularly, the embodiments relate to a method for producing a cBN thin film, including depositing cBN onto nanocrystalline diamond having controlled surface irregularity characteristics to improve the adhesion at the interface of cBN/nanocrystalline diamond, while incorporating hydrogen to a reaction gas upon the synthesis of cBN and controlling the feed time of hydrogen, so that harmful reactions occurring on a surface of nanocrystalline diamond and residual stress applied to cBN may be inhibited. The embodiments also relates to a cBN thin film structure obtained by the method.
2. Description of the Related Art
Cubic boron nitride (cBN) has physical properties similar to those of diamond which has the highest hardness and heat conductivity. In addition to this, cBN has higher oxidation resistance, high-temperature stability and reaction stability against iron-based metals as compared to diamond. Therefore, cBN is a material having high industrial applicability as a wear-resistant thin film for cutting tools, molds, or the like. Such a cBN thin film may be deposited by various processes including chemical vapor deposition or vacuum physical vapor deposition. However, the cBN thin film deposited by such processes is problematic in terms of its adhesion, and thus cannot be applied in practice. Weak adhesion of a cBN thin film results from the presence of an amorphous BN thin film layer and turbostratic BN (tBN) thin film layer formed at the initial stage of cBN thin film formation and high residual compression stress of the thin film. To solve such problems, attempts have been made to reduce the residual stress of the thin film or to enhance the adhesion by the insertion of an artificial intermediate layer.
Meanwhile, diamond has a lattice parameter similar to that of cBN, and thus may grow cBN on diamond at a high temperature of 900° C. or more. Based on this, there has been suggested a so-called heteroepitaxial film of cBN/diamond film that ensures excellent adhesion to diamond by allowing growth of a cBN thin film without formation of any weak intermediate layer such as tBN (U.S. Pat. No. 7,645,513). In the cBN/diamond composite film suggested from the U.S. Pat. No. 7,645,513, the diamond film corresponds to a microcrystalline diamond film synthesized through a chemical vapor deposition process. Such a diamond thin film has the following typical characteristics. First, the diamond thin film includes micrometer-sized diamond particles and, as shown in FIG. 1, has a distinct particle growth surface, thereby representing surface irregularities. Such surface irregularities adversely affect cBN deposition due to the reasons described hereinafter. Next, residual stress is applied to diamond itself. It is known that a diamond thin film has a residual stress of several GPas applied thereto. It is also well known that such residual stress causes a drop in adhesion between diamond and a matrix.
Now, the reasons why the surface irregularities of a diamond thin film adversely affect cBN synthesis will be described. The synthesis of a cBN thin film is accomplished by an ion beam deposition or sputtering process. An ion beam deposition process requires independent ion collisions having a predetermined level of energy and flux. A sputtering process requires ion collisions having a predetermined level of energy and flux from plasma (W. Kulisch and S. Ulrich, “Parameter spaces for the nucleation and the subsequent growth of cubic boron nitride”, Thin Solid Films 423 (2003) 183-195). In addition, during the synthesis of cBN, the cBN content of the resultant film, deposition rate, etc., significantly depend on the energy of ions. During a sputtering process, it is possible to control the energy of ions by modifying negative voltage applied to a substrate. Particularly, sputtering processes are used widely in various industrial fields, and uniform ion energy has to be ensured to perform stable and uniform deposition of cBN. Meanwhile, actual energy of ions is determined not by the average voltage applied to a substrate but by a local electric field, and electric field characteristics depend on surface shapes. In the case of a sharp surface, electric field is concentrated, resulting in an increase in ion energy. In addition, it is highly likely that ion flux depending on electric field distribution also increases. Therefore, when a surface has irregularities like a micrometer-sized diamond thin film, cBN deposition may be affected locally by the surface irregularities. Due to the above reasons, applications using a micrometer-sized diamond thin film as an intermediate layer for cBN deposition have several problems.
Meanwhile, there has been suggested a method for adding hydrogen to a reaction gas to reduce the residual stress (H.-S. Kim, J.-K. Park, W.-S. Lee and Y.-J. Baik, “Variation of residual stress in cubic boron nitride film caused by hydrogen addition during unbalanced magnetron sputtering”, Thin Solid Films 519 (2011) 7871-7874). It is highly likely that the method reduces the residual stress very simply to ensure adhesion. In addition, it is expected that combination of the method with an intermediate layer such as diamond provides adhesion enhancement as well as residual stress reduction, and thus is very useful in practice. However, hydrogen may be adsorbed easily to diamond. Thus, when hydrogen is added during the synthesis of cBN, it may react with a diamond surface so that the diamond/cBN adhesion is lower than originally expected, resulting in interruption of the growth of a cBN/diamond thin film.